1. Field of the Invention
The present invention relates to a composition for pneumatic tires and particularly to a tire tread obtained by vulcanizing said composition to provide pneumatic tires well balanced among five characteristic properties, i.e., tire rolling resistance, wet grip performance, wear resistance, steering stability and crack resistance and more particularly to provide pneumatic tires especially improved in rolling resistance and wear resistance.
2. Prior Art
In general tire treads, styrene butadiene copolymers of the straight chain type obtained by emulsion polymerization were used in most cases. Styrene butadiene copolymers obtained by solution polymerization were also used, but with respect to the relationship between styrene content and vinyl content, the conventional styrene-butadiene copolymers were less in vinyl content (the vinyl content here means 1, 2 structure butadiene content in the butadiene component) and furthermore were insufficient in the balancing among the above mentioned five major characteristics.
The inventors have provided, in the prior Japanese patent application No. 56-73954 (U.S. patent application Ser. No. 358,871, now abandoned), a rubber composition for pneumatic tires well balanced among five characteristics. Said rubber composition includes more than 30% of star shaped styrene-butadiene copolymers by solution polymerization with or without one or more types of natural rubbers or diene series synthetic rubbers, the composition value X obtained by the following equation of said styrene-butadiene copolymers being in the range of 26-39: ##EQU2## wherein BR is the butadiene component and the vinyl content means the content of 1, 2 structure butadiene in butadiene component.